1. Field of the Invention
The present invention relates to a method for manufacturing a zinciferous electroplated steel sheet excellent in press-formability.
2. Related Art Statement
A zinciferous electroplated steel sheet having a zinciferous electroplating layer comprising zinc or a zinc alloy formed on at least one surface thereof, has many advantages including an excellent corrosion resistance and a low manufacturing cost. The zinciferous electroplated steel sheet is therefore widely used as a steel sheet for an automobile body and as a steel sheet for a home electric appliance.
The zinciferous electroplated steel sheet has however a problem in that it has a press-formability inferior to that of a cold-rolled steel sheet. More particularly, frictional resistance of the zinciferous electroplated steel sheet against a forming die during the press forming is higher than that of the cold-rolled steel sheet against the forming die. As a result, when the zinciferous electroplated steel sheet is subjected to a severe press forming, peeloffs are produced in the zinciferous electroplating layer, and peeled-off pieces of the zinciferous electroplating layer adhere onto the surface of the forming die. When another zinciferous electroplated steel sheet is press-formed with the use of the forming die thus having the peeled-off pieces of the zinciferous electroplating layer adhering onto the surface thereof, serious defects such as flaws may easily be produced in the press-formed zinciferous electroplated steel sheet.
As zinciferous electroplated steel sheets which solve the above-mentioned problems, the following electroplated steel sheets each having a film for improving press-formability formed on the zinciferous electroplating layer are proposed:
(1) A zinciferous electroplated steel sheet having a powdering-preventive film, disclosed in Japanese Patent Provisional Publication No. 57-149,486 dated Sep. 16, 1982, which comprises: a steel sheet; a zinciferous electroplating layer formed on at least one surface of said steel sheet; and a powdering-preventive film in an amount within a range of from 0.2 to 20 g/m.sup.2 per surface of said steel sheet, formed on said zinciferous electroplating layer, which film comprises a macromolecular compound resulting from copolymerization of a monomer with .alpha.-.beta. ethylene unsaturated carboxylic acid, and a waxy substance in an amount within a range of from 1 to 40 wt. % relative to said macromolecular compound (hereinafter referred to as the "prior art 1").
(2) A zinciferous electroplated steel sheet excellent in press-formability, disclosed in Japanese Patent Provisional Publication No. 2-190,483 dated Jul. 26, 1990, which comprises: a steel sheet; a zinciferous electroplating layer formed on at least one surface of said steel sheet; and an oxide film comprising mainly zinc oxide, formed on said zinciferous electroplating layer (hereinafter referred to as the "prior art 2").
The above-mentioned prior arts 1 and 2 have however the following problems;
(1) It is necessary to provide a step of forming a film for improving press-formability on the zinciferous electroplating layer, thus resulting in a higher cost for the formation of this film.
(2) The film for improving press-formability, formed on the zinciferous electroplating layer, is scraped off during the press forming, and scraped-off pieces of the film adhere onto the surface of the forming die. It is therefore necessary to remove these pieces of the film adhering onto the forming die. Insufficient removal of the pieces of the film may easily cause production of flaws and other defects in the press-formed zinciferous electroplated steel sheet.
(3) The film for improving press-formability formed on the zinciferous electroplating layer exerts an adverse effect on the formation of a phosphate film and a paint film on the surface of the zinciferous electroplated steel sheet after the press forming. More specifically, a dense phosphate film and a dense paint film cannot be formed on the surface of the zinciferous electroplated steel sheet because of the presence of the film for improving press-formability, leading to easy degradation of the appearance and corrosion resistance of the zinciferous electroplated steel sheet having the phosphate film and the paint film formed thereon. Prior to forming the phosphate film and the paint film described above, therefore, it is necessary to completely remove the film for improving press-formability formed on the zinciferous electroplating layer. The removing operation of the film is not however easy and requires much time and labor, resulting in an increased cost.
Under such circumstances, there is a strong demand for the development of a method for manufacturing a zinciferous electroplated steel sheet excellent in press-formability, in which peeloffs are not produced in the zinciferous electroplating layer during the press forming, without forming a film for improving press-formability on the zinciferous electroplating layer, but such a method has not as yet been proposed.